6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



next 12 to 15 minutes we see the establishment of the assimilation 

 rate. The light is left on for 20 minutes, then is turned off (time = 

 35), and we see the immediate termination of assimilation and ap- 

 pearance of respiration with practically no time lag. This respiration 

 rate is the same as before the plant was illuminated. This sharp break 

 which has always been found under all conditions indicates the reality 

 of our belief that respiration under constant temperature and humidity 

 conditions proceeds during assimilation at the same rate as before or 

 after a period of illumination. If this were not the case, surely the 

 plants would have to readjust themselves to the new rate and that 

 readjustment would appear here as a curve. This method then throws 

 new light on the much-considered question as to the rate of respira- 

 tion during assimilation. The assimilation and respirational rates were 

 measured i hour and 40 minutes later and had the values shown in 

 brackets. 



The above example illustrates the response of a plant following a 

 long period of darkness. Let us interrupt it during the middle of its 

 day and see how it responds. Figure 5 shows the type of response 

 following a few hours exposure to light. Here the induction period 

 after 10 minutes previous darkness is only slightly over 2 minutes in 

 length. The sharp break occurs as always when the light is turned 

 off. There is a surprising similarity as to time between this induction 

 period for young wheat to that observed for algae by Warburg ( 1928, 

 pp. 341-345) and Van der Paauw (1932, pp. 595-598). In this figure 

 we see the induction period observed continuously for the first time. 

 Both Warburg and Van der Paauw used indirect integrational 

 methods. Figure 6, from data by Van der Paauw, shows this induc- 

 tion period for Hormidhim. The striking similarity between these 

 data for algae and those for young wheat plants in figure 5 shows 

 that we are dealing with a mechanism fundamentally the same in both 

 plants for carbon dioxide assimilation. 



In the work of Van der Paauw on the variation of induction with 

 temperature, we note that at a lower temperature more time is neces- 

 sary to establish the final assimilation rate. Figure 7 shows similar re- 

 sults for measurements made at 12° C. and 31° C. for young wheat 

 plants, i. e., a longer induction period at lower temperatures. Attention 

 is directed to the obvious difference in respiration at these two tempera- 

 tures. The assimilation rates would be about as different when cor- 

 rected for respiration. 



